I made a question at Stack Overflow long ago and also posted the same question in a non-Stack Exchange website (Xtreme VB Talk). The question was answered, Combining two PNG images into one image using .NET.
According to the answer, I shouldn't have cross-posted between Stack Overflow and XtremeVBTalk, because
it increases the probability that wires will get crossed and everyone will waste time.
But I'm not sure why is that wrong. You see, I simply see Stack Overflow and XtremeVBTalk as two different communities with different users. So posting the question in both sites would just give me more answers. And if I got the answer in one of them, I would report back to the other site and let everyone know (and perhaps someone else would be benefited as well). I really doubt it is a waste of time - the more input, the better. The fact that someone was a user in both websites is simply a coincidence.
I realize that Meta Stack Exchange question Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site? exists, but apparently it involves cross-posting within Stack Exchange sites, but this is about external websites instead.
I also realize that I was told that as "advice" (since my question was not flagged or anything), but well, I am now curious: Why would anyone consider this a bad practice?